I realize that mod remapping is done automatically, but why is this an issue? Or am I wrong in that it's an issue? The client just uses all the mods from the modpack and the server does too. Just to clarify, Bukkit plugins are serverside only which I think you realize.
Sorry, I should have been more explicit. I launched the (vanilla-with-FML) MineCraft CLIENT and got past the Mojang screen but when I selected Multiplayer I got an error screen asking for all of the missing mods that I assume should have been in “C:/Users/Name/AppData/Roaming/.minecraft/Mods”. I then ran the FTB launcher, selected the DireWolf20 modpack and the game ran fine.
Mods do not get installed into .minecraft unless you put them there yourself. FTB uses a separate directory for all of its minecraft instances. When you run FTB nothing is put into your .minecraft.
Aside from being easier to run and update, is there a compelling reason to use the FTB launcher rather than downloading all of the missing client-side mods for use with the vanilla-with-FML launcher?
No, there's no other reason besides convenience really. Because server and client side config files must match in addition to forge versions matching (typically), as well as mod versions having to match. You could set up your own launcher with source from certain github repositories but FTB is simply more convenient since all the mods and various config files have already been generated. If you want your own set of mods you could always set up a private pack through FTB as well. But any updates will only be pushed every Monday so that might be inconvenient for you.
In the case of multiverse, yes it looks like to do those things you would need the plugin. Although most of it could probably be accomplished with mystcraft and a ruins mod as well. But multiverse would be easiest. I don't have experience with multiverse on a mcpc server though I've seen some people run it. I just don't really know about its performance. Hopefully someone else can fill in here. If you do have trouble with portals from multiverse though, I think mystcraft linking books might be able to connect, but again, that would remove the economy aspect in that it costs nothing to use linking books.